Cargo: ignore BoringSSL's go.mod

This is changed by Go during build, so "cargo publish" detects a change
in the source while building and aborts.

Cargo.toml

diff --git a/Cargo.toml b/Cargo.toml
index af2dd52..9530165 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -15,6 +15,7 @@
     "deps/boringssl/crypto/**/*_tests.txt",
     "deps/boringssl/**/nist_cavp/*.txt",
     "deps/boringssl/fuzz/*",
+    "deps/boringssl/go.mod",
     "deps/boringssl/ssl/test/runner/*",
     "deps/boringssl/third_party/wycheproof_testvectors/*",
     "fuzz/*",